1. Field of The Invention
The present invention relates to an arrester in shape of a tank having a non-linear resistor the main component of which is a zinc oxide element.
2. Related Prior Art
An arrester, or lighting arrester, using a zinc oxide element has excellent characteristics, such as current-voltage linearity, discharge withstand current rating characteristics and chemical stability, and thus, it has been widely used in place of a conventional arrester utilizing series gaps and a silicon carbide non-linear resistor. In recent years, an arrester having further protective characteristics for use in a high potential system, such as 275 kV or 500 kV, has been developed and employed.
An arrester of the type described above is in a trend that the average stress (charging rate) in a system voltage always applied is raised for use. In order to ensure and maintain reliability for a long time, development of technology for uniformly assigning voltage for the purpose of uniforming assigning voltage for each zinc oxide element becomes significantly important.
A conventional tank-shape arrester will be first described with reference to FIGS. 7 and 8.
A non-linear element group 1 formed by stacking zinc oxide elements in series is accommodated in a cylindrical grounding tank 3 which is placed in a vertical attitude and in which an insulating medium 2, such as SF.sub.6 gas, exhibiting excellent insulating characteristics is enclosed, the non-linear element group 1 being disposed coaxially with the grounding tank 3. An axial end, i.e. the top end in the illustration, of the non-linear element group 1 is connected to a bus-line from a transforming station side through a high-potential conductor 5 supported by an insulating spacer 4. In the tank-shape arrester, a shield 6 having an umbrella-like shape is further disposed in the high potential side of the non-linear element group 1 and a ground potential portion is connected to the low potential side of the non-linear element group 1. Two or more annular ring-shaped shields 8 are disposed on the low potential side of the umbrella-shaped shield 6 through a plurality of, for example, four, connection support members 7 each having a narrow width in the circumferential direction so that voltage assignment to the zinc oxide elements in the non-linear element group 1 is uniformed.
Another example of a conventional tank-shape arrester is shown in FIG. 8, in which a circular arc-shaped shield 9 is, in place of the annular shield, connected to the shield 6 through a connection support member 7.
Although the tank-shape arresters of the types arranged as shown in FIGS. 7 and 8 are able to uniform the voltage assignment with a satisfactory accuracy for practical use to 500 kV class, a problem arises in that a required accuracy cannot be obtained to 1000 kV class which has been researched and developed at present time.
The reason for causing such problem will be described hereunder with reference to FIGS. 5A, 5B and 6. FIGS. 5A and 5B are views for the explaining the control of the potential distribution. Like reference numerals are added to elements or members corresponding to those shown in FIG. 8 and their descriptions are omitted.
In order to completely uniform the potential in the non-linear element 1, an electric current, which leaks to the grounding tank 3 serving as a grounding potential and which is the same as the charging current, is required to flow from the shield on the high potential side. Therefore, the following Equations (1) and (2) are held initially. EQU C(x).multidot.dx[1-V(x)]=Cs(x).multidot.dx.multidot.V(x) (1) EQU V(x)=1-x (2)
where C(x) is a capacitance between the high potential shield and the zinc oxide element at position x and Cs(x) is a capacitance between the zinc oxide element and the ground potential at position x. By arranging Equations (1) and (2), the following Equation (3) can be obtained. EQU C(x)/Cs(x)=1/(x-1) (3)
FIG. 6 is a graph expressing Equation (3). Namely, it is a graph showing capacitance distribution in an ideal state. That is, by realizing the shield shape satisfying the capacitance distribution as represented in Equation (3) and FIGS. 5A and 5B, a uniform voltage assignment in the axial direction of the non-linear element group 1 can be obtained even if the zinc oxide element has no capacitance.
However, it is difficult in actuality to completely realize the shield shape satisfying such characteristics as shown in FIG. 6. Therefore, a variety of approximated shapes have been suggested as exemplified by those shown in FIGS. 7 and 8. The zinc oxide element is provided with a function to serve as a dielectric substance having a relatively large dielectric constant in a state where the system voltage is always applied. Therefore, the effect of the self-electrostatic capacity of the zinc oxide element enables an approximated shield shape to restrict the voltage assignment to a satisfactory practical level depending upon the class of voltage (500 kV class).
Since the tank-shape arrester shown in FIG. 7 uses the annular shield 8, the capacitance C(x) between the non-linear element group 1 and the grounding tank 3 facing each other through the annular shield 8 is shielded to be approximately zero. Therefore, the value of C(x)/Cs(x) becomes excessively apart from the ideal state shown in FIG. 6. As a result, the potential distribution of the non-linear element group 1 is disordered. Therefore, the number of pieces disposed in series in the non-linear element group 1 increases as compared with the 500 kV class. As a result, the dispersion of the voltage assignment cannot be controlled to a satisfactory range for practical use if the shield shape as shown in FIG. 7 is employed in the 1000 kV class having a smaller self-electrostatic capacitance, thus being inconvenient.
In order to obviate such problem or defect, the prior art further provides an arrester having a rod-like or plate-like shield projecting diagonally. However, the arrester of this type involves a too complicated shield structure, and analysis thereof is hence made difficult. Therefore, an actual measurement is required whenever the structure of the non-linear element group 1 is changed. Thus, such an arrester cannot be used easily. Accordingly, a tank-shape arrester having a simplified shield structure as shown in FIG. 8 has been suggested.
It might be considered to employ a structure for the arrester to be adapted to a high potential system of 1000 kV class, the structure in which a plurality of, for example, four, parallel zinc oxide element groups in shape of columns are connected in parallel to serve as a non-linear element group because of the following two main reasons.
(1) In order to reduce the size of an equipment and a power transmission line, a very low limit voltage (a protection level) is set to the arrester, and in order to realize the low limit voltage, it is necessary to connect the zinc oxide element groups (columns) in parallel and to reduce a surge electric current flowing through each zinc oxide element column to lower the limit voltage.
(2) Since the diameter of the conductor in the power transmission line is enlarged and the number of the conductors is increased, the surge impedance is lowered and thus the load required for performing opening/closing operation becomes heavier. In addition, severer resistance against an excess voltage for a short time due to interruption of a load is required. Thus, a required energy resistance quantity becomes severer and, therefore, it becomes necessary to increase an energy resisting quantity by connecting the zinc oxide element columns.
It is important for the arrester of 1000 kV class to uniform the divided current flowing through each parallel column to a satisfactory level. In particular, the zinc oxide element may cause an imbalance of the divided flow if the current-voltage characteristics of each parallel column are not arranged accurately because the zinc oxide element has an excellent non-linearity. For example, the imbalance of the divided flow cannot be restricted to be within .+-.10% if the dispersion of the limit voltage for each parallel column is not controlled to be within .+-.0.2% as shown in the following Equation (4). ##EQU1##
Since the dispersion of the limit voltage for each zinc oxide element involves about .+-.10% in usual, for example, five, elements are combined as one block to control the dispersion to be about .+-.0.2%. The thus combined blocks are stacked so as to correspond to the rated voltage. The dispersion of the limit voltage is decreased in proportion to 1/n.sup.1/2 (n: integer) in an assumption of a normal distribution if the number of the zinc oxide elements in series is n. Therefore, since the arrester of the 1000 kV class comprising about 300 zinc oxide elements disposed in series provides a dispersion of about .+-.0.26% even if the elements are stacked randomly as expressed by the following Equation (5), it can be controlled practically. EQU 2%(.sqroot.300/5)=0.26% (5)
However, the asymmetrical arrangement of the shield of the arrester shown in FIG. 8 provides an imbalance of the potential distribution for each parallel column of the non-linear element group 1. Therefore, it is difficult to control the potential distribution of all parallel columns to be uniform. In order to overcome this problem, it is necessary to divide each parallel column into a plurality of blocks having an adequate number of elements and to mutually connect the parallel columns in each block. In this case, it is difficult to easily and precisely control the combination of the blocks, resulting in the causing of the imbalance of the divided flow, thus being disadvantageous in the discharge resistance.
The prior art further provides a structure in which a plurality of circular arc-shaped shields are disposed symmetrically with respect to a non-linear element group composed of a plurality of parallel columns. However, since a circular-directional free end of the circular arc shield becomes an excessively high electric field, the electric field must be relaxed by causing the circular-directional free end of the circular arc shield to have an adequate spherical surface. Therefore, it is difficult to manufacture an arrestor of such structure.
As described above, the conventional tank-shape arrester encounters a difficulty in uniforming the voltage assignment in the non-linear element group. In particular, if the arrester has a large capacity and uses a non-linear element group composed of a plurality of parallel columns, the imbalance in the divided current flow between columns cannot be easily prevented, thus being inconvenient.